Centrifugal cleaners have been known for decades. In a typical use of a centrifugal cleaner it is desirable to remove as many contaminants (rejects, debris) as possible while removing as little desirable material (accepts) as possible. Many different structures and implementation schemes have been designed to accomplish this desirable end result, however conventional cleaners still are not as effective as desired for many applications. For example in the separation of contaminants--such as pieces of plastic, particularly particles having a size of less than 0.5 mm.sup.2 --from conventional paper pulp furnish, even when four stages of centrifugal cleaner are utilized the effectiveness of contaminants removal is less than desired, and too much fiber is lost in the rejects stream.
According to the present invention, three distinct--but readily combinable--improvements are provided to conventional centrifugal cleaners which remarkably improve their effectiveness. Virtually all centrifugal cleaners have a generally hollow main body with a side wall having a generally decreasing conical taper from the top toward the bottom, a tangential inlet nozzle in the side wall near the body top for introducing fluid material to be cleaned, a top outlet nozzle extending downwardly into the body through the top and centrally located in the body, the bottom of the top nozzle located below the tangential inlet nozzle, and a bottom nozzle disposed in the side wall and generally concentric with the top outlet nozzle, and spaced from the tangential inlet nozzle. The improvements according to the invention relate to the configuration of the tangential inlet nozzle, the provision of a cylindrical section of the body in the side wall between conical upper and lower portions, and the particular construction of the bottom nozzle to define a particular rejects opening.
A typical tangential inlet nozzle is circular in configuration. It has been found according to the present invention that a circular configuration is far from ideal, causing turbulence and flow patterns which interfere significantly with the vortex action desirably introduced into the fluid by the tangential inlet. According to the present invention, a non-circular opening of smaller cross dimension than is typically utilized is provided. The inlet opening has no portion thereof which intersects the top nozzle--that is an extension from the straight leg of the D is substantially coincident with the exterior surface of the top nozzle. With such a configuration, for the same pressure drop there is higher velocity (and therefore better cleaning action) and increased throughput. A better free vortex is provided at the top of the cleaner, and since the type of turbulence and complicated flows typically ensuing when the flow of introduced fluid impacts the top nozzle is eliminated, the throughput is increased on the order of about 25%.
The second improvement according to the present invention is the provision of an interior section of the cleaner body which increases retention time. When retention time is increased, the time for the particles to "settle" is also increased, and thus the removal efficiency. According to the present invention, the body side wall of the cleaner comprises an upper portion that is conical and extends past the bottom of the top nozzle, and a conical bottom portion of the side wall which is disposed in operative association with the bottom nozzle. Between them is a means for increasing retention time within the cleaner, comprising a generally cylindrical center portion of the side wall.
The third improvement according to the invention is the most significant from the operational standpoint. By providing a particular design of bottom nozzle according to the present invention it is possible to eliminate multiple stages of multiple stage cleaners, to maximize the accepts portion, and minimize the rejects portion discharged from the cleaner, while effectively removing contaminants. The cleaner according to the present invention is particularly effective with contaminant particles having a size of less than about 0.5 mm.sup.2. The cleaner according to the present invention has a cleaning quotient greater than 0.5, and a rejects by weight performance of less than 5%, for conventional paper pulp furnish, remarkable advances compared to prior art cleaners not utilizing the invention.
The bottom nozzle according to the invention includes means for defining a rejects opening having a diameter that is about 25-45% of the internal diameter of the top outlet nozzle, e.g. roughly 1/3 (about 37%) of the internal diameter of the top outlet nozzle. Preferably it has a diameter that is about 25-45% of the internal diameter of the side wall at the rejects opening too, again typically roughly 1/3 the internal diameter of the side wall at the rejects opening.
The rejects opening is best formed in one of two ways. In the first embodiment the means defining the rejects opening comprises a substantially flat plate having a main top surface generally perpendicular to the top nozzle, and having an exterior diameter equal to the interior diameter of the side wall at that point along the body. In this embodiment rejects particles build up in about one or two minutes of operation to form an internal three dimensional parabolic surface tapering down to the rejects opening. According to a second embodiment of the invention, an insert is provided that already has the three dimensional parabolic configuration so that actual discharge of rejects begins almost immediately upon starting operation of the cleaner.
It is the primary object of the present invention to provide a centrifugal cleaner with enhanced effectiveness. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.